1. Field of the Invention
Aspects of the present invention relate to a data restoring method and a holographic storage data recording and/or reproducing apparatus which moves two-dimensional quantization data to an appropriate location using an alignment mark.
2. Description of the Related Art
Data is recorded in a holographic storage medium by interference between a signal beam and a reference beam. In optical holography, data is not stored on a surface of a recording medium, but is stored in a volume thereof. A signal beam interferes with a reference beam within the recording medium to generate a plurality of interference gratings referred to as a data page. The interference gratings change the optical characteristics of the reference beam, causing overlapping to occur. This process is referred to as multiplexing. To read data from the recording medium, a single reference beam is controlled to be incident on the recording medium under the same conditions as the conditions used during the data recording, thereby generating a diffraction beam having the stored data page. The diffraction beam is detected by a detection array, which extracts a stored plurality of data bits from a measured intensity pattern. The data page contains the data bits, which are also referred to as pixels. By overlapping data pages in the volume of the recording medium, data storage capacity is increased.
As shown in FIG. 1A, a hologram 100 is recorded using a signal beam S to carry data and a reference beam R. During recording of the hologram 100, the reference beam R and the signal beam S interfere with each other to generate an interference pattern, which is transferred to the hologram 100. During reproduction of the hologram 100, as illustrated in FIG. 1B, the original reference beam R is radiated onto the recorded hologram 100, and the recorded hologram diffracts the original reference beam R to generate the output signal beam S. At this time, if the reference beam R is different from the original reference beam R used during recording of data, the intensity or direction of a reproduced signal beam S is different from the intensity or duration of the original recorded signal beam S. Generally, as such a difference increases, the intensity of radiation is defined by a sinc function.
FIG. 2 is a schematic view of a conventional holographic data recording and/or reproducing apparatus 200. Referring to FIG. 2, a signal beam S is controlled to record a page having a plurality of pixels via a spatial light modulator (SLM) 220. The modulated signal beam S passes through an optical system including a polarized beam splitter 230 and a Fourier lens 240, and interferes with a reference beam R passing through a galvanometer scanner 260 and a scan lens 270 on a holographic storage medium 250. An interference fringe generated by this process is recorded on the storage medium 250. In terms of reproduction, when the reference beam R is controlled to be incident on the storage medium 250 where the interference fringe is recorded, the signal beam S is reproduced and detected by a charge-coupled device (CCD) 290 using the Fourier lens 280 due to a diffraction phenomenon. In this case, since different data is reproduced according to the depth and angle of the reference beam R during reproduction, several pages of data can be recorded and reproduced on the same location of the holographic storage medium 250.
A binary signal is recorded on the holographic storage medium 250 as follows. Two-dimensional binary data is generated in the SLM 220. The two-dimensional binary data is detected by a data detection device, such as the CCD 290, as two-dimensional quantization data having an intensity distribution according to optical intensity, and then is reconverted back into two-dimensional binary data. During this time, the two-dimensional quantization data has to be moved to a predetermined location in order to reproduce the two-dimensional quantization data detected by the CCD 290.
FIG. 3A illustrates two-dimensional binary data 310 transmitted by the SLM 220, and FIG. 3B illustrates two-dimensional quantization data 320 detected by the CCD 290. The two-dimensional binary data 310 illustrated in FIG. 3A is used to record data, and each data unit is classified into 0 or 1. The two-dimensional quantization data 320 illustrated in FIG. 3B is similar to the two-dimensional binary data 310 illustrated in FIG. 3A but is gradually shifted according to errors, reproducing locations and angles of various optical elements in the holographic data recording and/or reproducing apparatus 200. That is, after the two-dimensional binary data 310, portions of which have different optical densities, is transmitted through the holographic storage medium 250 and the optical system, a brightness difference between portions of the two-dimensional binary data 310 occurs, and the two-dimensional binary data is represented as a quantified numerical value, i.e., the two-dimensional quantization data 320, rather than the two-dimensional binary data 310. Thus, the reproducing location and angle of the two-dimensional binary data 310 are slightly tilted.
Accordingly, in order to restore the original binary data 310 from the two-dimensional quantization data 320 detected as described above, a signal calibration operation using various processes should be performed. The first process to be performed among the various processes is to move x and y coordinate values to a predetermined location, which is difficult.